Using the AmaSeis seismogram viewer and downloading - QuarkNet

Using AmaSeis… Page 15 of 69
Figure 19. Extracted seismogram from the selection shown in Figure 18.
3.5 Filtering seismograms with AmaSeis: Filtering is a digital signal processing operation
that is designed to enhance features of the seismogram that have certain frequency content.
Commonly, certain arrivals (P-waves, S-waves, surface waves, etc.) have more or less distinct
frequency characteristics and can be enhanced (or reduced) by filtering. Further, some types of
noise on seismograms are characterized by specific frequencies that can be reduced with
appropriate filtering. One of the most useful applications of filtering of seismograms is to
enhance the S-wave to better identify this arrival on the seismogram. To apply filtering, select
the Filter option from the Control menu in AmaSeis. A dialog box (Filter Dialog, Figure 20)
will appear. The filter settings shown in the dialog box in Figure 20 have been found to be
useful for enhancing the S-wave arrival. For example, for the Dominican Republic seismogram
shown in Figure 19, the filter cutoffs shown in Figure 20 were applied (enter the filter cutoff
frequencies and click on OK) resulting in the filtered seismogram shown in Figure 21. Note
that the S-wave (at about 04:58 time, UTC) is much more distinct in the filtered seismogram.
The S-wave is identified on the seismogram in Figure 21 by both strong relative amplitude and a
change in frequency (to a somewhat lower frequency, visible by the greater time between up
and down motions).